1. Field of the Invention
The present invention relates to a portable infusion pump, and in particular, to a portable infusion pump for supplying medicine such as a liquid having a high caloric value to a living body, for example.
2. Description of the Prior Art
A portable infusion pump which can be carried, for example, on a body of a patient and which transports medicine is required to be of a small size and of a light weight. Consequently, control circuitry for controlling a motor of the pump is advantageously configured in a simple structure to the extent possible.
A direct current (DC) motor is suitable for a small-sized apparatus; furthermore, since a DC motor can be easily controlled in motor speed, it is particularly advantageous for a portable infusion pump which needs to control the flow rate of a liquid to be supplied. In general, conventional control apparatus for controlling the flow rate of liquid uses a variable resistor to control the motor speed. However, the adjusting precision of such a variable resistor is relatively coarse and there does not exist a clear one-to-one correspondence between the position of an adjusting slider and the motor speed, which leads to an insufficient reproducibility. Namely, the correspondence cannot be fixedly determined between the flow rate, or a flowing volume per unit period of time, of the liquid transported by the pump and the position of the slider of the variable resistor, and hence a satisfactory reproducibility cannot be attained.
As a result, in the conventional apparatus for controlling the liquid flow rate, the scale marks of the control for adjusting the flow rate indicate only approximated targets, namely, it is difficult to indicate a digit representative of an appropriate setting value.
Moreover, when using a variable resistor, there may not exist a linear relationship between the positions of the adjusting slider and the speed of the motor. In order to design scale marks of the control for adjusting the flow rate to represent directly the flow rate, the scale marks are not distributed with a uniform interval between adjacent ones, which leads to a disadvantage that the adjusting operation cannot be easily achieved.
As well known, as a compact infusion pump for medicine to be used to supply medicine to an organism, there have been used a roller pump and a finger pump having a tube not exposed to the external environment thereof so as to prevent a chance of an infection. Since the pump is required to be of a small size, it is difficult to install in such an infusion pump an apparatus for actually measuring the flow rate.
As can be clear from this situation, in a case where such a compact infusion pump is used, it has been considerably difficult to measure the flow rate of nutritional solutions and other intravenous solutions actually flowing through the liquid transporting tube and further to obtain a setting value of the flow rate.